Mobile terminated communication from a source entity (calling party) to a destination device (called party) in a mobile communication system can be performed easily when the source entity knows an identifier of the destination device with which the destination device can be reached, such as a phone number.
There are situations however where the identifier with which the destination device can be reached is not known by the source entity.
Let us take the non-limiting example of the Evolved Packet System (EPS) architecture as defined in 3GPP TR 23.401 (GPRS enhancements for E-UTRAN access). In the EPS architecture, the Evolved Packet Core (EPC) network exists in several flavours. Depicted in FIG. 1 is the roaming EPC architecture with Local Breakout, which basically means that the IP address and the services in this configuration are provided by the Visited network. The IP address being hosted on a Packet Data Network Gateway (PDN GW or PGW), the latter also resides in the Visited network.
Another major EPC flavour (not shown here) is the “home routed” EPC architecture, which is used when the IP address and the services are hosted in the Home network.
EPC is a Packet Switched (PS) domain only architecture. All communications to and from a terminal are based on IP packet communication. The IP address assigned to the terminal can belong to the public or private address space. In the latter case, a Network Address Translation (NAT) device residing on or beyond the SGi reference point may be used for IP address translation.
When the terminal is assigned an IP address (public or private), the latter can be used immediately for client-server communications i.e. for communications that are always initiated by a client application running on the terminal.
However, barring some special cases of limited interest (e.g. statically assigned public IP addresses), this is not sufficient for enabling mobile terminated communications, such as “push” type communications (i.e. communications initiated by a server in the network towards the mobile device) or peer-to-peer communications (e.g. communications between two mobile devices).
If the IP address of the mobile device were known in advance by the server, then the latter could initiate Mobile Terminated (MT) communications with no problem. However, most of the time a new IP address is dynamically assigned to the mobile device every time it attaches to the network, the main reason being that it may be assigned by a different administration than the one owning the mobile user's subscription. For instance, in reference to the Local Breakout architecture in FIG. 1, the IP address is assigned by the Visited network (VPLMN), so it cannot be known in advance by the Home network (HPLMN).
Traditionally, the problem with MT communications has been approached in one of the following ways:                By using a separate channel (e.g. SMS) for delivery of a “push” stimulus to the terminal, triggering the latter to contact the server via client-server IP communication;        By overlaying a subsystem for peer-to-peer communications on top of EPS (e.g. the 3GPP's IP Multimedia Subsystem—IMS).        
While both approaches are used today in 3GPP standards, they may not be suitable in certain cases because they put additional complexity on the device in the form of additional communication channel (e.g. SMS) or additional client software (e.g. IMS). Moreover, both these approaches incur additional delay, which makes them less suitable for time critical delivery of mobile terminated messages.